THE CORRECT STRUCTURE OF THE ACID RHODO COMPLEX - A CLASSICAL COORDINATION COMPOUND. Pernille Harris, Henrik Birkedal, and Sine Larsen, Centre for Crystallographic Studies, Department of Chemistry, University of Copenhagen, Denmark

The acid rhodo chromium(III) compound has been known for more than a century. It is a dimeric complex with two Cr(NH₃)₅³⁺ moieties linked by a u-hydroxo group. The electronic and magnetic properties of crystalline [(NH₃)₅Cr-OH-Cr(NH₃)₅]Cl₅H₂O have been extensively studied (H. Riesen and H.U. Gudel, Inorg. Chem. 25, (1986), 3566) but these results were obviously at variance with the published crystal structures, assigned to the tetragonal space groups P4₂/mnm and P4_l2_l2. Making use of an extensive set of low temperature diffraction data we have acquired, what we believe is the correct structural description of this classical coordination compound. Our result is in accordance with the spectral and magnetic measurements. The structure has been solved in the orthorhombic space group P2_l2_l2₁ with cell dimensions a= 16.157(5) Å, b= 16.164(5) Å and c= 14.766(3) Å. The crystals emulate tetragonal symmetry by having a reflection twin in the (110) plane. There are two crystallographically different dimers, i.e. four chromium sites, in the asymmetric unit. The four chromium sites are connected by pseudo-tetragonal symmetry, corresponding to a cell with c' = c/2. This means that reflections with l= 2n + 1 are systematically weak, a feature that gave rise to further complications in the structure determination. The Cr-O-Cr angle of the two dimers is 154.8(3)deg. and 152.8(4)deg., respectively; this small difference is also apparent from ESR-measurements. The angle between the Cr-O-Cr planes of the two dimers is 90.2(1)deg. - in agreement with optical measurements. It has not been possible for us to locate hydrogen atoms. This is most probably due to the twinned nature of the data. The final model has an R-value of 0.059 for 6949 reflections having F₀ > 4 [[sigma]](F₀).